1. Field of the Invention
The present invention relates to extended interaction klystrons having increased power, efficiency, and bandwidth, and more particularly, to a novel assembly for tuning klystron cavity resonance that additionally suppresses undesirable harmonic RF energy.
2. Description of Related Art
Linear beam tubes are used in sophisticated communication and radar systems which require amplification of an RF or microwave electromagnetic signal. A conventional klystron is an example of a linear beam microwave amplifier. A klystron comprises a number of cavities divided into essentially three sections: an input section, a buncher section, and an output section. An electron beam is sent through the klystron, and is velocity modulated by an RF electromagnetic input signal that is provided to the input section. In the buncher section, the electrons that have had their velocity increased gradually overtake the slower electrons, resulting in electron bunching. The traveling electron bunches represent an RF current in the electron beam. The RF current induces electromagnetic energy into the output section of the klystron as the bunched beam passes through the output cavity, and the electromagnetic energy is extracted from the klystron at the output section. An output waveguide channels the electromagnetic energy to an output device, such as an antenna.
Klystron amplifiers having large instantaneous bandwidth are provided by use of multi-cavity output circuits. The multi-cavity output circuits, known as extended interaction output circuits (EIOC), have the advantage that a higher level of impedance across a greater bandwidth can be achieved. The higher impedance enables better matching with the electron beam, leading to greater efficiency of operation. An EIOC used to produce high power microwave energy with large instantaneous bandwidth is referred to as an extended interaction klystron (EIK), and can be used to produce power over bandwidths in excess of 10%. Examples of high performance EIOCs are disclosed in U.S. Pat. Nos. 4,931,695, to Symons, and 5,304,942, to Symons et al.
Despite the advantages of EIKs in terms of power, efficiency, and instantaneous bandwidth, there are certain significant drawbacks. To insure that the klystron resonates within the proper frequency range, it is advantageous to include cavity tuners that are widely adjustable (in excess of 10% tuning range), rugged (i.e., able to survive numerous cycles and high power levels), and that do not adversely affect cavity R/Q (a figure of merit which directly relates to the gain-bandwidth product where R equals the shunt resistance of the cavity and Q equals the quality factor). Klystrons are often provided with an inductive tuning mechanism for adjusting the resonance of the klystron.
A typical inductive tuner for a klystron comprises a thin metallic diaphragm which serves as a movable wall of the resonant cavity. Adjustment of the diaphragm position alters the inductance of the cavity, thus changing its resonance characteristics. The diaphragm also serves to provide a part of the vacuum envelope for the klystron, and is therefore sealed to the klystron cavity around the entire circumference of the diaphragm. As a result, the range of motion of the diaphragm is limited due to its attachment to the cavity. The limited range of motion of such diaphragms is inadequate for the wide range of adjustment necessary for broad bandwidth klystrons. Moreover, excess movement of the diaphragm often causes cracking with the subsequent loss of vacuum within the klystron.
Notwithstanding the klystron tuning difficulties, a secondary problem with high efficiency klystrons is elevated harmonic levels within the cavity. Harmonics are an inevitable, but undesirable, by product of high efficiency klystron design. The harmonics are undesirable since energy devoted to the harmonics detracts from the signal purity and power of the fundamental frequency. Load mismatches at the output waveguide further aggravate this situation by reflecting back impedance levels which cause increased harmonic energy to be created. It is desirable, therefore, to damp out the harmonics within the klystron itself so that the levels are diminished sufficiently to prevent conditions outside the klystron, such as impedance mismatches at the output waveguide, from causing a problem.
Accordingly, it would be desirable to provide an apparatus for use with an EIK that enables a broad range of cavity tuning, and that suppresses undesired harmonic resonances encountered within the cavity. It would be further desirable to provide an apparatus having the above characteristics, while being relatively simple to design and cost effective to fabricate.